In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, the use of network nodes employing multi-antenna techniques has the potential to increase the performance of the communications network a whole order of magnitude. By spreading out the information to be communicated and the total transmission power wisely over multiple antennas, several gains can be harvested, such as increased spectral efficiency, reduced interference, increased link reliability in terms of diversity, and so on.
Communications networks employing network nodes with more than one transmit antenna can be categorized into two main groups based on how each antenna element can be controlled; digital beamforming and analogue beamforming, respectively.
With digital beamforming (DgBF) the transmitted signal from each antenna can be controlled arbitrarily. In the equivalent baseband model, this corresponds to sending a complex-valued symbol with arbitrary amplitude and phase from each of the antennas.
With analogue beamforming (AnBF) the transmitted signal from each antenna cannot be set to any arbitrary value due to simplified hardware. In the equivalent baseband model, the transmitted complex-valued symbols from the antennas have the same amplitude and only differ from one another in a fixed phase, linearly.
Precoding is a generalization of beamforming to support multi-stream (or multi-layer) transmission in multi-antenna wireless communications. Digital beamforming may be regarded as giving more freedom in selecting appropriate precoders but generally requires more advanced, and possibly more expensive, hardware. It is foreseen that in future radio systems, a combination of digital beamforming and analogue beamforming can be used. For example, an antenna array comprising antenna panels (herein also referred to as ports) can be individually controlled with AnBF and the antenna panels can be jointly controlled with DgBF. In other words, for the antennas in each antenna panel, AnBF can be applied, and for the groups of antennas (belonging to each antenna panel), DgBF can be applied.
In order to support multiple-input multiple-output (MIMO) communications, for example, in Long Term Evolution Frequency-Division (LTE FDD) the wireless device can be configured to report which precoding and rank the wireless device estimates is best to use. To limit the overhead of the reporting only a set of precoders are possible to report for the wireless device. The precoders are associated with reference signals transmitted by the radio access network node. The set of available precoders is denoted a codebook, which is reported using an index and a rank, e.g. a Precoder Matrix indicator (PMI) and a Rank Indicator (RI). The PMI thus refers to reference signals transmitted by the radio access network node and may hence be regarded as a means for the wireless device to report the reference signals.
However, there is still a need for an improved handling of reference signal reporting.